1. Field of the Invention
The present invention relates to a zoom lens that can be used, for example in an image pickup apparatus.
2. Description of the Related Art
A small zoom lens having a high zoom ratio is useful for an image pickup apparatus (e.g., camera, a video camera and a digital still camera) that uses a solid-state image sensor.
In this type of camera, various types of optical members such as a low-pass filter and a color correction filter are disposed between a rearmost portion of a lens and a solid-state image sensor.
Thus, it is useful that a zoom lens for use in these types of cameras has a relatively long back focus.
An optical system in which a long back focus can be secured includes a negative lead type zoom lens in which a lens unit having a negative refractive power is located in the forefront.
Meanwhile, in the case of a color camera that uses a solid-state image sensor for a color image, a zoom lens having good telecentric characteristics on an image side is useful in order to prevent a color shading from occurring.
Conventionally, a three-unit zoom lens is known which includes a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power, in order from an object side to an image side, which has a small front lens element, and whose image side is telecentric (Japanese Patent Application Laid-Open No. 2001-66503 and Japanese Patent Application Laid-Open No. 2001-281545).
In addition, a three-unit zoom lens is known whose first lens unit includes two lenses, namely a negative lens and a positive lens, which uses a glass material having a high refractive index for the negative lens, and which includes a lens whose one surface is aspheric (U.S. Pat. No. 7,061,685).
The market has strongly desired that a zoom lens for use in a video camera or a digital camera is small and has a high zoom ratio.
In the above-mentioned three-unit zoom lens including lens units respectively having negative, positive, and positive refractive powers, a moving stroke of the second lens unit increases as the zoom ratio becomes higher. As a result, the entire length of a lens system becomes long and it is difficult to miniaturize the lens system and obtain a high zoom ratio at the same time. Accordingly, most three-unit lenses have a zoom magnification (zoom ratio) of about 3×.
In addition, the thickness of an image pickup apparatus is reduced by using a retraction method in which an interval of the lens units is shortened in a non-photographing state.
In this regard, in order to further reduce the thickness of an image pickup apparatus, it is necessary that the thickness of the lens units is reduced.
U.S. Pat. No. 7,061,685 discusses a three-unit zoom lens whose first lens unit includes a negative lens and a positive lens so as to reduce the thickness of the lens unit and to correct or reduce aberration.
In a three-unit zoom lens, a configuration of a first lens unit having a negative refractive power is significant in correcting or reducing variation of aberration occurring during zooming and in miniaturizing the entire lens system.